Industry has long utilized various spray coating systems for the application to surfaces of coating materials such as paints, adhering powders and the like. Two great disadvantages are inherent to most of these systems. First, much of the material to be applied is wasted, owing to failure to reach the surface or adhere to the surface. Second, irregular surfaces were often not adequately covered; recesses, for example, have proven especially difficult to coat.
Various improvements of the basic spray system have been developed, most with the goal of decreasing the amount of coating material wasted and/or improving the evenness of coverage of the coating material. Possibly the most significant of these has been the use of electric fields for influencing the trajectory of particles of the coating material as they move in the direction of the surface to be coated. Such systems to date have depended upon the imposition of static electric fields, for example, between the element from which the coating material issues and the surface to be coated, to charge the particles of the coating material and to subsequently control their trajectories. These are known as electrostatic spray systems.
Prior electrostatic systems have been generally of three types. In the first, paint is sprayed past an electrode to which a D.C. voltage is applied. A portion of the paint particles accept a charge from the electrode and are then electrically attracted to an oppositely charged workpiece. In another, a conductive-paint system, having a bulk paint supply, paint pump and delivery conduits, is maintained at the electrostatic charging potential, as, for example, 50 kv to 100 kv. The charged paint system must be protected from access by the operator, complicating servicing of the supply, replenishment of paint and changing of paint color. Also, the electrical energy stored in the charged paint system presents a danger of fire or explosion in the event of a short circuit. The third system grounds the paint supply and spray gun while maintaining the article being coated at an electrical potential of the order of 100 kv. The paint particles are not electrically charged on formation at the gun and the higher efficiency of paint deposition associated with charging the paint particles is not achieved.
Electrostatic spray systems, though a significant improvement over electrically neutral systems, do not adequately coat many highly irregular surfaces. Many electrostatic spray coating operations rely on a final manually directed, non-electrostatic application of coating material to provide acceptable coverage of recessed areas of the surface being treated.
The present invention is directed to overcoming one or more of the problems as set forth above.